In the metals recovery and refining industry generally, the type of metal recoverable from a solvent is dependent upon the size of the electrochemical window of the solvent in which the metal is dissolved, and from which purification and recovery is taking place. In aqueous solutions, this is governed by the electrochemical window of water or supporting electrolyte. This limits the recovery, purification and electroplating of metals on to surfaces from aqueous solution to those metals whose electrode reduction potentials are more positive than the cathodic limit of the aqueous solution. In acidic aqueous solution, metal ions would not be recoverable where their electrode reduction potentials are more negative than that of the H3O+ ION. Recovery of metals with electrode reduction potentials more negative than H3O+, means that non-aqueous (aprotic) solvents are required. There are a number of aprotic solvents which are used. There are a number of non-protic solvents which are used, These are often molten salts and, for instance, aluminium is industrially purified electrochemically by electrolysis of Al2O3 dissolved in molten cryolite Na3AlF6. Other aprotic media include the organic solvents, such as acetonitrile, benzene and toluene.
There exists two well developed processes which use molten salts for the reprocessing of irradiated nuclear fuel. The Argonne National Laboratory electrometallurgical treatment (ANL—EMT) process and the Dimitrovgrad State Scientific Centre, Research Institute of Atomic Reactors (SSC—RIAR) process both use molten salts at high temperatures (773 and 1000K respectively). The ANL process treats the spent nuclear fuel by a process called electrorefining in which current flow is used to oxidise a uranium anode to form uranium ions in the molten salt electrolyte. At the cathode the uranium is reduced and electrodeposited as uranium metal. The SSC—RLAR process uses chemical oxidants (chlorine and oxygen gases) to react with powdered UO2 fuel to form higher oxidation state compounds such as UO2Cl2 which are soluble in the molten salt. At the cathode the uranium compounds are reduced to UO2, which forms a dendritic deposit.
The disadvantage of these processes is that these molten salts are typically mixtures of salts which are liquid only at high temperatures and this causes inherent disadvantages in a reprocessing plant, in particular, as a result of the challenges posed in the engineering of the process and the materials of construction.
Ionic liquids free of molecular solvents were first disclosed by Hurley and Wier in a series of U.S. Pat. Nos. 2,446,331, 2,446,339, 2,446,350. In general terms an ionic liquid is a salt, a mixture of salts, or a mixture of components which produce a salt or a mixture of salts, which melts below or just above room temperature. (As used herein, the term “salt” means an entity comprising entirely of cationic and anionic species). Such liquids are known as “ionic liquids” although this term is sometimes used for salts which melt at relatively high temperatures. In this specification, the term “ionic liquid” refers to a salt which melts at a temperature of up to 100° C.
Co-pending patent application PCT/GB99/00246 discloses a method for reprocessing spent nuclear fuel which comprises dissolving the spent fuel or constituent parts of the spent fuel in an ionic liquid to substantially separate fissile material from other components of irradiated fuel. Also disclosed is the subsequent treatment of the resulting ionic liquor, either by solvent extraction or electrochemical treatment to recover the dissolved uranium and plutonium.
Whilst the methods described in PCT/GB99/00246 are suitable for general use and, in particular, for use in nuclear fuel reprocessing, it has previously been thought that an electrorefining process, which avoids the need for an initial chemical dissolution step, requires the use of a high temperature molten salt electrolyte. If fuel is chemically oxidatively dissolved, there is less control over the species which are dissolved during this step. All those species which will be oxidised by the oxidising agent added will enter into the solution. Because the oxidising agents and conditions are aggressive, most species will dissolve except for species such as the noble metals.